Aerosols Affect Pacific Trade Winds

Aerosols from volcanoes and human emissions affect the Pacific trade winds, according to a new paper. This graphic from the paper shows time series of the Pacific trade-wind index from: two reanalysis data sets (a); from a historical simulation driven by all external forcing agents (b); and from a simulation that held volcanic and human aerosol levels constant at pre-industrial levels (c). Courtesy: Nature Climate change and the authors.

Pacific trade winds have recently intensified to a degree that is unprecedented in the the past century and this change can not be explained by natural variability of the climate system. A new study investigates the role of volcanic and human related aerosols using computer models and reanalysed historic data.

Volcanic and anthropogenic sulphate aerosols have been implicated by researchers in changes in the intensity of the trade winds observed over the last two decades.

The recent increase in trade wind intensity, the strongest observed in the past century, is related to a warming of western North Pacific sea surface temperatures, according to a paper published in Nature Climate Change. Around one-third of the Pacific trade wind intensification between 1991 and 2010 can be attributed to sulphate aerosols, according to climate scientists Chiharu Takahashi and Masahiro Watanabe from the University of Tokyo, authors of the paper.

Sea-surface temperatures (SSTs) in the western North Pacific decreased from the 1930s to the early 1990s before increasing rapidly over the past two decades. Volcanic sulphate aerosol emissions have been low since the Pinatubo eruption in 1991 although human-related activity has continued to release sulphate aerosols into the atmosphere, particularly the western Pacific Asian regions.

An accompanying article in Nature Climate Change, by Mark Collier of the CSIRO climate research organsation in Australia, discusses this research and states that the “main conclusion is that the natural aerosol-induced SST warming accounts for a substantial fraction of the wind intensification in the central Pacific; the remainder, especially since the mid-1990s, is mostly attributable to natural cooling in the eastern Pacific and physical exchanges with the Atlantic”.

The authors warn in their paper: “If sulfate aerosol cooling is strengthened again in the near future by large volcanic eruptions or further increases of anthropogenic emissions from Asia, it could cause regional sea-level rises and enhanced drying over the Maritime Continents, both of which would seriously impact the Pacific islands”.

Abstract

The Pacific trade winds, coupled with the zonal sea surface temperature gradient in the equatorial Pacific Ocean, control regional sea levels, and therefore their trend is a great concern in the Pacific Rim. Over the past two decades, easterly winds have been accelerated in association with eastern tropical Pacific cooling. They may represent natural interdecadal variability in the Pacific and possibly explain the recent global warming hiatus. However, the intensification of the winds has been the strongest ever observed in the past century, the reason for which is still unclear. Here we show, using multiple climate simulations for 1921–2014 by a global climate model, that approximately one-third of the trade-wind intensification for 1991–2010 can be attributed to changes in sulfate aerosols. The multidecadal sea surface temperature anomaly induced mostly by volcanic aerosols dominates in the western North Pacific, and its sign changed rapidly from negative to positive in the 1990s, coherently with Atlantic multidecadal variability. The western North Pacific warming resulted in intensification of trade winds to the west of the dateline. These trends have not contributed much to the global warming hiatus, but have greatly impacted rainfall over the western Pacific islands.